1. Field of the Invention
The present invention relates to an image forming apparatus such as a digital printer or a digital copying machine using electrophotographic process with modulation of laser beam.
2. Description of the Prior Art
In a digital image forming apparatus using electrophotographic process, a photoconductor is exposed with a laser beam. The laser beam is modulated according to multi-level digital data (or read data of a document) for reproducing a half-tone image. Thus, an electrostatic latent image is formed on the photoconductor in correspondence to the read data of a document. The latent image is developed to form a toner image, which is transferred to and fixed on a paper to reproduce the image of the document. In order to stabilize the reproduction of image, the maximum density is controlled to be kept constant. That is, the density of a standard toner image formed on the photoconductor is detected and the intensity of laser beam, the surface potential of the photoconductor or the like is adjusted.
Further, gradation correction of multi-level read data is important especially for a half-tone image or for a color image. In general, the image density level reproduced is not exactly proportional to the exposure level of laser beam due to various factors such as the photosensitivity characteristic of photoconductor, the toner characteristic, the environment and the like. Such nonlinearity is called as gradation characteristic (gamma characteristic), and it substantially degrades the fidelity of a reproduced image. Then, the output characteristic of laser power is corrected in order to realize linear characteristic. This is called a gradation correction (gamma correction). That is, for a multi-level read data, the output power is increased at low gradation levels, while it is decreased at high gradation levels, so as to make the density of a reproduced image proportional to the gradation of read data.
When a latent image is formed on a photoconductor, the quantity of exposure light of laser beam is modulated for each pixel according to read data corrected as explained above. The laser beam may be modulated on the intensity of laser beam under constant pulse width or on the pulse width thereof under constant intensity. In the intensity modulation, the intensity of laser beam is modulated for each pixel according to the read data after gradation correction while keeping the exposure time of laser beam constant for a pixel. In the pulse width modulation, the exposure time (pulse width) of laser beam is modulated for each pixel according to the read data after gradation correction while keeping the intensity of laser beam constant. In the two modulation methods, multi-gradation can be represented for each pixel. However, the output density characteristic (gamma characteristic) of a reproduced image becomes nonlinear against read data (quantity of exposure light). Therefore, the above-mentioned gamma correction is needed to obtain a desired reproduction characteristic.
The gamma correction becomes large especially at low densities (high-light portion), and the reproduction characteristic is liable to change. Further, even small noises may be reproduced. If the amount of gamma correction is small, the stability of reproduced image becomes better, and the image can be reproduced well at low densities. Therefore, it is desirable that the amount of gamma correction becomes small.